1. Field of the Invention
The invention concerns an arrangement to adjust the homogeneity of a basic magnetic field in a magnetic resonance apparatus.
2. Description of the Prior Art
In a magnetic resonance apparatus it is necessary to homogenize the basic magnetic field by shimming. The shimming ensues passively, for example, with use of ferromagnetic materials.
The shimming of the basic magnetic field (also called the B0 field) is necessary due to inaccuracies in the production of the basic field magnet. Moreover, external influences at the installation site (such as iron reinforcements in the building, etc.) lead to magnetic field distortions.
In general, plates that are known as shim plates are used for shimming. These are fashioned as small iron plates or discs, for example, and are inserted into carriers formed as drawers and are positioned therein. These carrier drawers are known as shim trays.
Multiple channels that are fashioned to accommodate the shim trays (and thus serve for the arrangement or positioning of the shim plates) are fashioned at the gradient coil carrier of the magnetic resonance scanner.
In a first pass, for the adjustment of the desired homogeneity the basic magnetic field is measured without additional shim plates. If an insufficient homogeneity of the basic magnetic field exists, the positions of individual shim trays at which shim plates must be inserted in order to improve the homogeneity is then calculated.
The shim plates are inserted and fixed at these positions. The magnetic field is then reestablished and its homogeneity is measured again.
It is necessary to secure the respective shim trays or carrier drawers against movement with elaborate means. For example, this ensues using specially designed bolt connections.
This increased expenditure is necessary because magnetic forces that could loosen the bolt connection act on the shim trays. Forces of multiple hundreds of Newtons act on a single shim tray, and therefore on the associated bolt connections.
The shim trays must additionally be secured against vibrations that are caused by the gradient coil in operation.
An additional disadvantage results from the fact that the gradient coil support and the gradient coil system attached thereto are composed of different materials that exhibit different thermal properties and expand differently upon heating.
Gradient coil systems and gradient coil carriers form a module composed of epoxy resin, glass, copper, etc. These materials become heated up to 80° C. in operation and expand differently, such that the homogeneity of the magnetic field is also affected due to relative position changes of the shim elements.